1. Field of the Invention
The present invention relates to methods and apparatus using attenuation of penetrating radiation to determine a concentration of material in an object. More specifically, the present invention relates to methods and apparatus using X-ray attenuation to determine a concentration of dopant in a soot preform used to form optical waveguide fibers (xe2x80x9coptical fibersxe2x80x9d).
2. Description of the Related Art
An optical fiber typically includes a cladding made of pure silica (SiO2) and a core made of silica doped with germania (GeO2). The germania dopant alters the refractive index of the silica in the core. Portions of the core often contain different concentrations of germania, resulting in different refractive indexes along the diameter of the core. The distribution of refractive indexes along the diameter of the core (i.e., the refractive-index profile) determines operating characteristics of the optical fiber.
The optical fiber can be formed by a conventional process known as outside vapor deposition (xe2x80x9cOVDxe2x80x9d). Generally, the OVD process involves forming a soot preform by burning a gaseous mixture to produce soot containing silica and germania, successively depositing layers of that soot onto a mandrel rod to form a core portion of the soot preform, burning a gaseous mixture to produce soot containing only silica, and successively depositing layers of that soot onto the core portion to form a cladding portion of the soot preform. The soot preform is consolidated by sintering to form a glass blank. An optical fiber is drawn from the glass blank. The concentrations of germania in the soot layers forming the core portion primarily determine the concentrations of germania along the diameter of the core of the resulting optical fiber.
Japanese Patent Application No. 59-106803 (Hara) and U.S. Pat. No. 4,618,975 (Glantschnig) disclose techniques that use X-ray attenuation to nondestructively evaluate the concentrations of germania in soot preforms. Both approaches measure X-ray attenuation at two energies. Hara""s scheme relies upon the fact that the dopant (Ge) to matrix (Si) attenuation ratio changes with X-ray photon energy. Hara""s scheme is not particularly sensitive for soot preforms, however, because the ratio is nearly constant over any practical X-ray energy range. Glantschnig""s method is based on the fact that the ratio of dopant attenuation (absorption) to density attenuation (scattering) changes with X-ray photon energy. Like Hara""s ratio, Glantschnig""s ratio is nearly constant over an energy range practical for soot preforms. Thus, Glantschnig""s method confounds density changes with dopant concentration changes.
As embodied and broadly described herein, the invention comprises a method of determining a concentration of dopant in soot that constitutes at least a portion of a soot preform used to form an optical waveguide. The method includes the steps of measuring the weight of the soot preform, measuring a thickness parameter of the soot preform, irradiating the soot with penetrating radiation, detecting intensity of penetrating radiation passing through the irradiated soot, and determining the concentration of dopant based on the measured weight, the measured thickness parameter, and the detected intensity of penetrating radiation.
Another aspect of the present invention comprises a method of determining a concentration of dopant in first and second segments of soot that constitute at least a portion of a soot preform used to form an optical waveguide. The method includes the steps of measuring the weight of the soot preform after the first segment of soot has been deposited on the soot preform, measuring a thickness parameter of the soot preform after the first segment of soot has been deposited on the soot preform, measuring the weight of the soot preform after the second segment of soot has been deposited on the soot preform, measuring a thickness parameter of the soot preform after the second segment of soot has been deposited on the soot preform, irradiating the second segment of soot with penetrating radiation, detecting intensity of penetrating radiation passing through the second segment of soot, determining the concentration of dopant in the second segment of soot based on the detected intensity of penetrating radiation passing through the second segment of soot and the measured weight and thickness parameter of the soot preform after the second segment of has been deposited on the soot preform, irradiating the first and second segments of soot with penetrating radiation, detecting intensity of penetrating radiation passing through the first and second segments of soot, and determining the concentration of dopant in the first segment of soot based on the detected intensity of penetrating radiation passing through the first and second segments of soot and the measured weight and thickness parameter of the soot preform after the first segment of soot has been deposited on the soot preform.
Yet another aspect of the present invention includes an apparatus for determining dopant concentration in soot that constitutes at least a portion of a soot preform used to form an optical waveguide. The apparatus comprises a weight-measuring device that measures the weight of the soot preform, a thickness-parameter-measuring device that measures a thickness parameter of the soot preform, a radiation source that irradiates the soot with penetrating radiation, a radiation sensor that detects intensity of penetrating radiation passing through the soot, and a determination device that determines a concentration of dopant in the soot based on the measured weight and thickness parameter, and the detected intensity of penetrating radiation.
A particularly preferred embodiment of the invention quantifies the soot density profile by successively measuring preform weight and preform diameter during soot deposition, measuring X-ray attenuation of the preform (during or after soot deposition), and computing dopant concentration profile from the solution of an attenuation equation.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.